Analysis Of Cloud Optical Characteristics And Retrieval Of Main Cloud Parameters For Satellite Polarized Remote Sensing

Cloud has a significant impact on climate change,however at present the understanding of the mechanism of cloud influence on climate is insufficient.The accurate retrieval of cloud parameters can provide reliable input parameters for the General Circulation Models(GCM).It will help to understand the role of clouds in climate change.The directional polarization camera(DPC)boarded on China's GF-5 satellite that has been successfully launched.DPC can obtain multi-angle intensity radiation and polarized radiation of cloud,which provide more effective information for cloud parameter retrieval and improve cloud parameter retrieval ability.Studying on the retrieval of both cloud optical thickness(COT)and cloud top height(CTH)are developed basing on DPC characteristics.The main work is implemented as following aspects:(1)Retrieval COT base on 670nm and 865nm bands of DPCThe sensitivity of the reflectance and polarized reflectance of 670nm and 865nm bands to COT were evaluated by vector radiative transfer model RT3.The results show that the COT can be retrieved from the reflectance of the two bands.The effects of surface reflectance,aerosol optical depth(AOD),cloud scattering model and cloud effective radius on the retrieval results of COT were simulated.In the simulation,ice cloud scattering model was assumed to be GHM(General Habit Mixture),SC(Solid Columns),ASC(Aggregates of Solid Columns),water cloud scattering model was assumed water droplet model.The simulation results show that the surface reflectance has great influence on the COT retrieval results,and the aerosol model has little influence on it.The AOD has great influence on COT retrieval results for thin clouds,while has less influence on thick clouds.For COT retrieval,different cloud scattering model and the cloud effective radius will obtain different COT retrieval results.Based on the above analysis,COT lookup table was constructed and COT retrieval algorithm based on the lookup table was designed.Over land,COT was retrieved by 670nm band.Over ocean,COT was retrieved by 865nm band.Using the French POLDER3 data to retrieve ice cloud COT which ice cloud was assumed to be GHM?SC and ASC model.The retrieval results were compared with POLDER3 COT products.The results show that the retrieval results of the ASC model have a good consistency with POLDER3 ice cloud COT products,and the retrieval results of the ASC model are almost not affected by the variation of the cloud effective radius.Therefore,the ASC model is suggested to be used as the scattering model of ice cloud for DPC to retrieve ice cloud COT.Ice clouds and water clouds over land and over ocean were retrieved based on POLDER3 data,the retrieval results were comparedwith the POLDER3 COT products.The COT retrieval results of ice clouds and water clouds over land and over ocean are all in good agreement with POLDER3 COT products.(2)Retrieval CTH based on polarized reflectance difference between 490nm and 865nm bandsThe difference of the atmosphere top polarized characteristics between 490nm and 865nm bands is given by theoretical analysis.Based on analysis results,it is feasible to use the polarization reflectance difference to retrieve CTH.The sensitivity of the polarized reflectance difference to the different cloud effective radius,COT,the AOD above cloud layer and the pressure of cloud top were evaluated by RT3.The results show that,for a specific scattering angle range(water cloud is 80° to 120°,GHM ice cloud model is 80° to 140°),the polarized reflectance difference is not sensitive to cloud effective radius,AOD above the cloud layer and COT when COT is greater than 3.However,it's sensitive to the pressure of the cloud top.According to the results of sensitivity analysis,the CTH retrieval algorithm based on polarization reflectance difference lookup table is proposed in this paper,and appropriate parameters were selected to construct a polarization reflectance difference lookup table.Using POLDER3 data to retrieve CTH by the CTH retrieval algorithm proposed in this paper.Retrieval results were compared with POLDER3 Rayleigh pressure products and MODIS cloud top pressure products.The results indicate that compared with the official POLDER3 algorithm,the polarized reflectance difference look-up table method proposed in this study had a better consistency with MODIS products,and could applied to a wider scattering angle range,obtaining more CTH data for ice cloud.(3)Retrieval COT based on 763nm and 765nm bands of DPCThe relationship between oxygen transmittance ratio at 763nm and 765nm band and pressure was simulated by MODTRAN5,and the fitting formula was given by simulation data.The influence between different atmospheric modes on the calculation of pressure results is analyzed.The influence of aerosol over cloud layer on the apparent pressure retrieval result by the fitting formula is analyzed by MODTRAN5 and RT3.The results show that when the cloud reflectance is small,the scattering of atmospheric molecules and aerosols above cloud layer make the apparent pressure retrieval results have greater errors.Therefore,the apparent pressure is suggested to be retrieved by using 763nm and 765nm band when the COT is greater than 3.The influence of surface contribution on the apparent pressure retrieval was theoretically analyzed.The correction method of obtaining cloud top pressure was given by analysis results.The retrieval algorithm of CTH based on the reflectance of 763nm and 765nm band was designed.The retrieval experiment was carried out by using POLDER3 data,and the retrieval results were compared with the retrieval results based on the polarizing reflectance difference method and the MODIS product.The results show that,compared with the CTH retrieval results based on the polarization reflectance difference method,the COT retrieval results based on the 763nm and 765nm bands are better relevance with the MODIS products,and can obtain more CTH products,however the retrieval accuracy are more poor.The research work of the dissertation can provide valuable reference for the retrieval method,retrieval parameter setting and cloud scattering model selection for COT and CTH retrieval of DPC payload of GF-5 satellite in China.